Cleaning method using a defluxing agent

ABSTRACT

The defluxing agent for flux residue after soldering contains an acid (preferably an organic acid, and particularly an acid stronger than abietic acid; for example, acrylic acid, acetic acid, propionic acid, benzoic acid) and an organic solvent (for example, xylene, benzyl acetate, methyl α-hydroxyisobutyrate, cyclohexanone, methyl β-methoxyisobutyrate), and if necessary it further contains a monohydric alcohol, a surfactant and a corrosion inhibitor. Rinsing is preferably performed after the cleaning, using a solvent which is miscible with the defluxing agent, in order to completely remove the acid. There is also disclosed a cleaning apparatus which may be generally used for this and other cleaning.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a defluxing agent, a cleaning methodand a cleaning apparatus, and especially it relates to a defluxingagent, cleaning method and cleaning apparatus for non-chlorine-typedefluxing after soldering.

The production and use of chlorine-type organic solvents which depletethe ozone layer are recently being ever more regulated by thelegislatures of participant nations to the Montreal Protocol. Sincechlorine-type organic solvents have been used in the past as defluxingagents for cleaning flux after soldering, there has been a greatacceleration in the development of defluxing agents which do not usechlorine-type organic solvents.

2. Description of the Related Art

During the production of electronic circuit parts by soldering, thepost-solder flux has conventionally been cleaned for removal using1,1,1-trichloroethane.

1,1,1-trichloroethane not only has a high ability to dissolve rosin, themain component of post-solder flux, but it also has a low toxicity andno inflammation point, giving it superior safety as a defluxing agent,and it has thus been indispensable for the cleaning of flux.

Since 1,1,1-trichloroethane is to be completely banned in 1996, fromthat time on the cleaning of post-solder flux during the production ofelectronic circuit parts by soldering will become more difficult, andbecause the long-term reliability of the electronic circuit componentswill be reduced as a result of corrosive substances contained in theflux, the development of a substitute defluxing agent for1,1,1-trichloroethane has become an urgent issue.

With the prolonged heating during soldering the acid components of therosin contained in the flux react with the solder to cause chemicalreactions such as the production of salts (metal carboxylic acid salts),etc., thus becoming insoluble to the solvent, and therefore thesubstitute defluxing agent for 1,1,1-trichloroethane must not simplydissolve the rosin prior to heating but must also dissolve the alteredflux components which have reacted due to heat.

Nevertheless, because in the past most research has centered onchlorine-type solvents such as 1,1,1-trichloroethane, nonon-chlorine-type solvents which dissolve heat-altered flux have beenknown.

DISCLOSURE OF THE INVENTION

Defluxing agent and Use thereof

(1) The present invention provides, with the purpose of overcoming theabove mentioned problems, a defluxing agent characterized by containingan acid and an organic solvent.

Flux consists of rosin (which consists mainly of abietic acid andderivatives thereof), an activator (normally an amine hydrochloride orthe like) and an organic solvent, and after it is applied to the area tobe soldered it is heated for soldering, and the residual flux must becleaned away lest it become a cause of metal corrosion and impair theoverall appearance.

For simple dissolution of the rosin, there are known a variety ofsolvents including monohydric alcohols such as methyl alcohol, ethylalcohol, n-propanol, isopropanol, n-butanol, etc.; esters such as ethylacetate, isopropyl acetate, etc.; polyhydric alcohol derivatives such asethylene glycol monomethyl ether, ethylene glycol monoethyl ether,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,ethylene glycol dimethyl ether, ethylene glycol diethyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc.,and the like. However, the monohydric alcohols, esters and polyhydricalcohol derivatives mentioned here cannot dissolve heat-altered fluxafter soldering.

In recent years, various defluxing agents comprising non-chlorine-typeorganic solvents have been developed and marketed, but like the abovementioned solvents, they have all had the disadvantage of leavingresidues to one degree or another after the cleaning of heat-alteredflux.

Furthermore, in Japanese Unexamined Patent Application (KOKAI) NO.63-56353 there is proposed a method of cleaning with a fluorinatedhydrocarbon after passage through hydrochloric acid, nitric acid or thelike, but since the degree of cleaning is insufficient, there is stillthe problem of the production of residues. This is thought to be due tothe residual components which cannot be thoroughly removed since they donot mix with the acidic aqueous solution used for the initial immersioncleaning or the fluorinated hydrocarbon used for the subsequentimmersion.

We the present inventors, as a result of diligent research, havediscovered surprisingly that a defluxing agent which includes an acidand an organic solvent very satisfactorily dissolves even rosincomponents which have been altered by heat during a long period ofsoldering. The dissolution mechanism of the altered rosin components bythis defluxing agent is not clear, but it is presumed to be thefollowing. That is, it is thought that because an acid and an organicsolvent are present as a mixture in the defluxing agent according to thepresent invention, metal carboxylic acid salts believed to be the causeof reduced solubility of the altered rosin dissociated by the acidcomponent, and the dissociated species become solubilized in the organicsolvent component, thus making possible complete cleaning with leavingno residue.

The type of acid and the type of solvent used in admixture therewith inthe defluxing agent may be freely selected depending on the purpose ofuse. Also, the acid and organic solvent may each be mixtures of 2 ormore components.

The acid to be used here is not particularly restricted, but acidsstronger than abietic acid are preferred because they allow easycleaning at room temperature, and also because they have a moreexcellent cleaning effect since most flux has abietic acid or aderivative thereof as a major component, causing the heat-altered fluxto contain abietic acid salts of the metal. The acid stronger thanabietic acid is preferably one which has an acid dissociation index(inverse log value of pK_(a) : acid dissociation constant) of 6 or lessas a standard in aqueous solution.

Also, organic acids have higher compatibility with organic solvents thando inorganic acids, and also have the advantage of less metal (solder)corrosion. Organic acids including carboxylic acids are preferred foruse. As examples there may be mentioned acetic acid, acrylic acid,benzoic acid, formic acid, propionic acid, butyric acid, isobutyricacid, pivalic acid, valeric acid, isovaleric acid, caproic acid,2-ethylbutyric acid, caprylic acid, 2-ethylhexanoic acid, oleic acid,citric acid, succinic acid, cinnamic acid, abietic acid, stearic acid,oxalic acid, malonic acid, maleic acid, tartaric acid, sebacic acid,phthalic acid, etc.

Particularly preferred organic acids are acrylic acid, acetic acid,propionic acid and benzoic acid. Acrylic acid and propionic acid haveparticularly suitable acid strengths, while acetic acid is inexpensiveand benzoic acid is preferable from the point of view safety (and odor).

Furthermore, the organic solvent to be used in combination with the acidis not particularly restricted so long as it is a component which doesnot create a phase separation with the above acid, and from the point ofview of safety for the environment, etc. and drying speed aftercleaning, it preferably contains no halogen elements such as chlorine orfluorine and has a boiling point of 50° C.-250° C. at one atmosphericpressure.

As examples there may be mentioned aliphatic hydrocarbons such ashexane, heptane, octane, nonane, decane, dodecane, undecane, etc.;aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene,mesitylene, naphthalene, tetralin, naphthaline, etc.; alcohols such asmethyl alcohol, ethyl alcohol, n-propanol, isopropanol, n-butanol,isobutanol, pentanol, hexanol, heptanol, etc.; phenols such as phenol,cresol, xylenol, etc.; ethers such as dipropyl ether, dibutyl ether,dihexyl ether, anisole, phenetole, dioxane, tetrahydrofuran, etc.;ketones such as acetone, methyl ethyl ketone, pentanone, hexanone,methyl isobutyl ketone, heptanone, diisobutyl ketone, cyclohexanone,acetophenone, etc.; esters (Here, the substituent which forms and esterwith the above acid is preferably a C₁ -C₁₀ hydrocarbon.) such as ethylformate, methyl acetate, benzyl acetate, butyric acid esters, isobutyricacid esters, hydroxyisobutyric acid esters, isovaleric acid esters,benzoic acid esters, citric acid esters, succinic acid esters, cinnamicacid esters, abietic acid esters, stearic acid esters, oxalic acidesters, malonic acid esters, maleic acid esters, tartaric acid esters,sebacic acid esters, phthalic acid esters, etc.; nitrogenous compoundssuch as acetonitrile, amines, etc.; sulfuric compounds such asdimethylsulfoxide; and compounds with two or more functional groups suchas 2-methoxyethanol, 2-ethoxyethanol, diethylene glycol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, diethylene glycol,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc.

In addition, as a result of recent progress in high density mountingonto circuit boards for electronic components, plastic parts andadhesives have come to be widely used in the process of assembly of thecircuit boards. Here, the plastic parts or adhesives of the circuitboards sometimes have problems of swelling and dissolution as a resultof the organic solvent mixed with the acid. Also, attempts to suppressthis effect on the parts often reduces the cleaning properties of thesolvent. Particularly preferable as solvents which have no effect onsuch parts and retain strong cleaning properties are toluene, xylene,benzyl acetate, methyl α-hydroxyisobutyrate, methylβ-methoxyisobutyrate, cyclohexanone, methyl acetate, amyl acetate,isopropyl alcohol and their derivatives.

The composition ratio of each of these solvents and acids may beadjusted depending on the purpose, and the acid is preferably used at0.01-50 parts by weight, and more preferably at 0.1-20 parts by weight,while the solvent in addition to the acid is preferably used at 50-99.99parts by weight, and more preferably at 80-99.9 parts by weight. If theamount of the acid is too low then the cleaning properties will beimpaired, and if too high there will be a high possibility of an adverseeffect on the solder joints and other metal parts. A particularlypreferable concentration is 1-10 parts by weight of the acid and 90-99parts by weight of the solvent.

Furthermore, it is suitable to add a monohydric alcohol as part of theorganic solvent, since this will have the effect of improving thecompatibility and improving the gloss of the soldering. It is preferablyadded in an amount of 5-50 parts by weight. If the amount of themonohydric alcohol added is too low the above effect will be minimal,and if too high the cleaning properties will be impaired.

(2) In addition, the present inventors have, as a result of diligentresearch, surprisingly discovered that particular effectiveness isachieved with an organic solution containing 50-95 parts by weight of aC6-C20 aromatic solvent and 5-50 parts by weight of a C3C15 ester,C2-C15 ketone and/or C1C15 alcohol solvent, or 50-90 parts by weight ofbutyl acetate and/or amyl acetate either alone or as a total and 5-50parts by weight of a C6-C20 aromatic solvent and 5-50 parts by weight ofa C3-C15 ester, C2-C15 ketone and/or C1-C15 alcohol solvent, and thatdefluxing agents containing these organic solvents are capable ofsatisfactorily dissolving the heat-altered rosin components even withoutthe acid.

The dissolution mechanism of the altered rosin components by thisdefluxing agent is not clear, but it is presumed to be the following.That is, it is thought that the main essential component, an aromaticsolvent in which the rosin itself has a high solubility, or inexpensivebutyl acetate or benzyl acetate which provides both high solubility anda high degree of safety for humans, combined with the further additionof a polar component, the C3-C15 ester, C2-C15 ketone and/or C1-C15alcohol solvent for dissociation of metal carboxylic acid salt which isbelieved to be the cause of the lowered solubility of the heated rosin,results in the dissociation of the metal salt, and the dissociated rosincomponents are then dissolved by the above mentioned main solvent todisplay the superior cleaning properties. Also, by adding an organicacid to the defluxing agent it is possible to further promote thedissociation of the metal carboxylic acid salt, and thus further improvethe cleaning properties.

The reason for the above limits to the carbon numbers is that if theyare below the given ranges there will be inconveniences of use such aslowering of the solubility and lowering of the flash point, thusincreasing its danger, while on the other hand if the carbon numbers areabove these ranges there will also be inconveniences of use such aslowering of the solubility and lowering of the volatility, thusincreasing the time required for drying.

The type of the main solvent, polar component and acid used in thedefluxing agent may be freely selected depending on the use.

As a result of investigation of the solubility of the heat-altered fluxby the present inventors, it was discovered that the solubilityparameter (SP) of the mixed solution should be 8.0-11.0, and preferably8.5-10.0, while preferably the dispersion force solubility parameter(δ_(d)) is 7.3-8.7 the polarity solubility parameter (δ_(p)) is 0.5-4.2and the hydrogen bonding solubility parameter (δ_(h)) is 1.0-9.5.Regarding δ_(d), δ_(p) and δ_(h), see p. 114 of the "Bonding Handbook(2nd Edition)", published 1982 by the Nihon Bonding Association.!

The aromatic solvent to be used as the main component may be benzene,toluene, xylene, ethylbenzene, cumene, mesitylene, naphthalene,tetralin, benzyl acetate, dibenzyl ether, dodecylbenzene, acetophenone,methyl benzoate, ethyl benzoate, or the like. Particularly preferableare xylene, because it is inexpensive, and tetralin, dibenzyl ether,dodecylbenzene and benzyl acetate, because they are inexpensive and havea high inflammation point. Also, the polar solvent to be added to themain component aromatic solvent or butyl acetate or amyl acetate, is notparticularly restricted so long as it does not create a phase separationwith the above mentioned components, and from the point of view ofsafety for the environment, etc. and drying speed after cleaning, itpreferably contains no chlorine and has a boiling point of 50° C.-250°C. at one atmospheric pressure. As examples there may be mentionedalcohols such as methyl alcohol, ethyl alcohol, n-propanol, isopropanol,n-butanol, isobutanol, pentanol, hexanol, heptanol, etc.; phenols suchas phenol, cresol, xylenol, etc.; ethers such as dipropyl ether, dibutylether, dihexyl ether, anisole, phenetole, dioxane, tetrahydrofuran,etc.; ketones such as acetone, methyl ethyl ketone, pentanone, hexanone,methyl isobutyl ketone, heptanone, diisobutyl ketone, cyclohexanone,acetophenone, etc.; esters (Here, the substituent which forms and esterwith the above acid is preferably a C₁ -C₁₀ hydrocarbon.) such as ethylformate, methyl acetate, ethyl acetate, propyl acetate, hexyl acetate,butyric acid esters, isobutyric acid esters, hydroxyisobutyric acidesters, alkoxyisobutyric acid esters, isovaleric acid esters, benzoicacid esters, citric acid esters, succinic acid esters, cinnamic acidesters, abietic acid esters, stearic acid esters, oxalic acid esters,malonic acid esters, maleic acid esters, tartaric acid esters, sebacicacid esters, phthalic acid esters, etc.; nitrogenous compounds such asacetonitrile, amines, etc.; sulfuric compounds such asdimethylsulfoxide; and compounds with two or more functional groups suchas 2-methoxyethanol, 2-ethoxyethanol, diethylene glycol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, diethylene glycol,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc.

In addition, as a result of recent progress in high density mountingonto circuit boards of electronic components, plastic parts andadhesives have come to be widely used in the process of assembly of thecircuit boards. Here, the plastic parts or adhesives of the circuitboards sometimes have problems of swelling and dissolution as a resultof the organic solvent mixed with the acid. Also, attempts to suppressthis effect on the parts often reduces the cleaning properties of thesolvent. Particularly preferable as solvents which have no effect onsuch parts and retain strong cleaning properties are toluene, xylene,benzyl acetate, dibenzyl ether, dodecylbenzene, acetophenone,cyclohexanone, methyl β-methoxyisobutyrate, methyl α-hydroxyisobutyrate,isopropyl alcohol and their derivatives.

The composition ratio of each of these solvents and acids may beadjusted depending on the purpose, and the acid is preferably used at0.01-50 parts by weight, and more preferably at 1-10 parts by weight. Ifthe amount of the acid is too low then the cleaning properties will beimpaired, and if too high there will be a high possibility of an adverseeffect on the soldered and other metal parts.

(3) Also, as mentioned above, since the above mentioned organic solventsdissolve heat-altered rosin even without an acid, the present inventionalso provides the following defluxing agents which contain no acid.

That is, a defluxing agent characterized by using as a constituentcomponent thereof an organic solvent mixture containing 50-95 parts byweight of an aromatic solvent of 6-20 carbon atoms and 5-50 parts byweight of at least one solvent selected from ester solvents of 3-15carbon atoms, ketone solvents of 2-15 carbon atoms and alcohol solventsof 1-15 carbon atoms, and containing no chlorine or fluorine; and

a defluxing agent characterized by using as a constituent componentthereof an organic solvent mixture containing 50-90 parts by weight ofbutyl acetate and/or amyl acetate and 5-50 parts by weight of at leastone solvent selected from ester solvents of 3-15 carbon atoms, ketonesolvents of 2-15 carbon atoms and alcohol solvents of 1-15 carbon atoms,and containing no chlorine or fluorine.

The most suitable system is one which uses a mixture containing 50-90parts by weight of xylene, 10-50 parts by weight of cyclohexanone and/ormethyl β-methoxyisobutyrate, and 0.5-10 parts by weight of acetic acid.Obviously it more preferable to add an acid thereto.

In addition, the defluxing agent according to the present inventionpreferably contains no water since there is a high possibility that itwill have an adverse effect on the metal part, but depending on the usethere is virtually no problem if it contains water in an amount of 2.0wt % or less.

A corrosion inhibitor may be added if necessary to minimize corrosion ofthe soldered or other metal. Metal phosphates, silicates, chromates,sulfonates, nitrites, vanadium-containing acids, and ammonium salts;organic phosphoric compounds such as alkyl phosphites, triarylphosphates; C₁ -C₁₀ alkylamines; thiourea, sorbitan monooleic acidesters, and benzotriazole, etc. are preferably used. The amount thereofto be added is preferably 10 ppm-10%. If too little is added thecorrosion inhibiting effect will be small, while if too much is addedthe cleaning properties will be reduced.

Furthermore, if a metal with a high susceptibility to corrosion ispresent in the part to be cleaned, then a dehydrating agent may be addedto the acid-added defluxing agent in order to prevent corrosion. Thedehydrating agent is not particularly restricted so long as it has adehydrating effect, but preferred for use are phosphorous pentaoxide,magnesium perchlorate, a silica gel or molecular sieve, borontriacetate, anhydrous copper sulfate, chrome (III) acetate, aceticanhydride, and the like.

A surfactant may be added to the defluxing agent if necessary. Thepresence of the surfactant improves the cleaning properties by loweringthe surface tension of the agent and facilitating permeation of theagent into the minute sections. There are no particular restrictions onthe type of the surfactant so long as it is surface active, but it ispreferable to use a cationic surfactant or an anionic surfactant suchas, for example, a carboxylic acid salt, sulfonate salt, sulfuric acidester, phosphoric acid ester or the like, a nonionic surfactant such aspolyethylene glycol, a polyhydric alcohol or the like, an amphotericsurfactant, fluorine-type surfactant, etc. The amount to be added ispreferably 1 ppm-1%, because if it lower it will lower the surfaceaction and if higher it will impair the cleaning properties.

The temperature of the defluxing agent may be room temperature, and ifnecessary the temperature may be raised to above room temperature forthe cleaning.

The method of cleaning using this defluxing agent is not particularlylimited, but it is preferable to use the immersion method, showermethod, spray method, paddle method, bubble method, ultrasonic method,etc.

The viscosity of the defluxing agent used is preferably 10 cp or lower,and especially 3 cp or lower, in order to allow easy dispersion andincrease the cleaning properties.

Also, as mentioned previously, the surface tension is preferably 40dyn/cm or lower, and especially 30 dyn/cm or lower, in order tofacilitate permeation.

Cleaning or defluxing method

(1) As mentioned above, it was discovered that by combining an acid withan organic solvent in the defluxing agent according to the presentinvention, it is possible to dissolve therein even heat-altered rosinand to immerse a circuit board in an acid-containing defluxing agent fora long period of time (or repeatedly) with no problems; however, sincethe lingering of the acid on the circuit board which is exposed to theair after cleaning is not preferred since it becomes a cause of metalcorrosion, it was found to be possible to easily remove the acid andthus minimize the metal corrosion by cleaning (rinsing) with a liquid(containing no acid) which readily mixes with the defluxing agent(particular the organic acid component) after the cleaning with theabove mentioned acid-containing defluxing agent.

Here, the organic solvent itself which was used in the acid-containingdefluxing agent is a representative example of what may be used as therinse liquid, and in addition to other organic solvents which may beused in the defluxing agent, water itself may be used so long as itmixes with the defluxing agent.

A surfactant may also be added to the rinse liquid if necessary.

Furthermore, as a result of recent progress in high density mountingonto circuit boards of electronic components, plastic parts andadhesives have come to be widely used in the process of assembly of thecircuit boards. Here, the plastic parts or adhesives on the circuitboards sometimes have problems of swelling and dissolution as a resultof the liquid used for rinsing.

In these cases, the solvent used to adjust the effect on these parts ismost preferably an aromatic-type solvent such as toluene or xylene, amonohydric alcohol-type solvent such as isopropyl alcohol, an ester-typesolvent such as butyl acetate, or water.

The rinse liquid may employ two or more types of solvent in admixture.Also, rinsing with a first liquid may be followed by rinsing with asecond liquid. The temperature of the rinse liquid may be roomtemperature, or the temperature may be raised as necessary.Alternatively, the rinsing may be performed by vapor.

The method of rinsing is not particularly restricted, but it ispreferable to use the immersion method, shower method, spray method,paddle method, bubble method, ultrasonic method, etc.

Incidentally, the residual acid concentration on a circuit board ispreferably 5 μg/in² or less of residual ion in terms of NaCl.

(2) Also, when mixtures of two or more organic compounds (particularlyorganic solvents) are used as the defluxing agent to substitute for1,1,1-trichloroethane, the difference in the heats of evaporation of theindividual organic solvents has been a problem in that, if the defluxingagent is used in an open system such as in the defluxing process,evaporation (vaporization) begins preferentially from the organicsolvent with the lower heat of evaporation, and thus the composition ofthe defluxing agent is altered with time.

Here, as an aspect of the present invention, granular or cylindricalfloating bodies are arranged on the top surface of the defluxing agentso that a method may be provided which allows minimizing of alterationsin the composition of the solution of the defluxing agent consisting ofa mixture of a plurality of types of organic solvents, and thusstabilizes the defluxing of the circuit board. The floating bodiespreferably cover the entire surface of the agent or solution, and theirshape is spherical, and they preferably consist of a mixture of spheresof differing diameters which are preferably hollow (light) and haveelectric conductivity (for an anti-static effect).

This cleaning method is not limited to the defluxing agent comprising anorganic solvent and an acid as mentioned above.

Cleaning apparatus

Since almost all of the defluxing agents to be substituted fortrichloroethane are inflammable, measures must be taken for theprevention of fires, and of these measures the most widely employed arethose which maintain the concentration of the inflammable gas within theexplosion limit by local gas discharge. However, with fire-preventionmeasures by local discharge of the gas, it is actually difficult toextinguish possible ignitions because of the large amount of air whichis constantly being supplied, while a large amount of discharge causesdirt to flutter, promoting clinging of dust to the member to be washed,and further if the amount of gas discharge is increased then the amountof evaporation also increases making the process less economical, whilealso raising the degree of atmospheric pollution.

Here, as an aspect of the present invention, these problems are overcomeby a cleaning apparatus in which a solvent is used to remove organicmatter and/or dirt clinging to a member to be cleaned, characterized byhaving a partitioned structure wherein the space in which the cleaningprocess is effected is an enclosed space isolated from the outside air,and having means for introducing nitrogen gas into the above enclosedspace and means for discharging the nitrogen gas, with the abovenitrogen gas-introducing means and discharging means capable of beingcontrolled to maintain a constant pressure differential between theabove enclosed space and the outside air.

In addition to the decrease in the concentration in the inflammable gas,it becomes more difficult to burn because of the reduction in the oxygenconcentration. Even a slight lowering of the oxygen concentration willresult in a lessening of the danger of explosion and combustion, but itis preferably adjusted to 10 vol % or less. The cleaning area is coveredand isolated from the outside air and nitrogen gas is introducedtherein, but it need not be nitrogen gas that is substituted for theair, although nitrogen is the most inexpensive. However, a pressuredifferential occurs inside the casing as a result of the introductionand discharge of the nitrogen gas. Cleaning apparatuses are generally onthe order of a few meters large, making the surface area of the casing atotal of as much as 10 or more square meters, and therefore even a smallpressure differential exerts a powerful force on the entire casing. Forexample, even with a pressure differential of 0.1 atmospheres = 100 hPathe pressure on the entire casing becomes 10 or more tons, and thiscreates the need for a strong, pressure-tight structure which isexpensive and heavy, making the device impractical.

This problem may be overcome by having means for introducing nitrogengas into the covered interior and means for discharging the nitrogen gasfrom the covered interior, and by controlling the above nitrogengas-introducing means and discharging means so that the internalpressure differential is kept at, for example, hPa or lower.

Function

The defluxing agent which comprises an acid and an organic solventsatisfactorily dissolves even rosin components which have been partiallyaltered by heat during soldering, for purification of the solderedportion.

By modifying the type and mixing ratio of the organic solvent to bemixed with the acid, the rosin and corrosive ionic substances residingin soldered portions are purged without exerting an adverse effect onthe plastic parts or adhesives of the circuit wiring board.

Furthermore, by using as the main component an aromatic solvent in whichthe rosin itself has a high solubility, or butyl acetate or benzylacetate which provides both high solubility and a high degree of safetyfor humans, and further adding thereto an ester-, ketone- and/oralcohol-type solvent as a polar solvent, and preferably an organic acid,for dissociation of the metal carboxylic acid salt, the metal is thusdissociated and the rosin components are subsequently dissolved by theabove mentioned main solvent to allow satisfactorily dissolution of eventhe rosin components which have been partially altered by heat duringthe soldering, thus purifying the soldered portion.

In this manner, complete cleaning of flux with a non-chlorine-typesolvent becomes possible.

In addition, by using a non-acid-containing liquid for removal (rinsing)of the defluxing agent after cleaning, it is possible to remove the acidcontained in the defluxing agent and thus to minimize corrosion of themetal parts on the circuit board.

By covering the top surface of the cleaning liquid with granular orcylindrical floating bodies, the evaporation (vaporization) of thecleaning liquid is minimized, and changes in the composition of theliquid are prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an IR analysis chart for flux residue heated together withsolder.

FIG. 2A is a photograph of the appearance of a soldered portion of asubstrate after cleaning with a commercially available defluxing agent,and FIG. 2B is a photograph of the appearance of a soldered portion of asubstrate after cleaning with a defluxing agent of the presentinvention.

FIG. 3A and 3B are drawings of cleaning apparatuses.

FIG. 4 is a graph showing the results of placing floating bodies on thecleaning bath.

FIG. 5 is a front cross-sectional view of a cleaning apparatus.

FIG. 6 is a side cross-sectional view of a cleaning apparatus.

FIG. 7 is a drawing showing the pressure-regulating mechanism at the N₂intake opening of a cleaning apparatus.

FIG. 8 is a drawing showing the pressure-regulating mechanism at thedischarge opening of a cleaning apparatus.

EXAMPLES Reference Example

Three types of solder (In-40Pb, Sn-37Pb, In-48Sn) and commerciallyavailable Flux MH-820V (product of Tamura Kaken Co.) heated at 215° C.for 25 minutes were cleaned with IPA, but a portion of the flux residueremained as uncleanable.

This flux residue was dissolved and extracted with tetrahydrofuran (THF)and analyzed. The results were as follows.

(1) As may be seen in the IR analysis shown in FIG. 1, the IPA insolublematter contained a large amount of metal carboxylic acid salts whichwere not contained in the IPA soluble matter.

(2) The residual components contained metals, halogens, etc. (XPSanalysis).

(3) No increase in the molecular weight of the residual components wasobserved, and no polymerization reaction of the rosin occurred by thelong-term heating (GPC analysis).

Furthermore, when the surfaces of the three types of solder weresubjected to elemental analysis after flux cleaning with THF, thefollowing became clear.

(4) When In-40Pb and Sn-37Pb solder are used, the portions with residualreflown flux have extremely lower amounts of In and Sn in comparisonwith the portions with no residual flux. Conversely, the distribution ofPb is greater in the portions with residual flux (because the number ofPb atoms per unit volume is relatively increased). Also, with In-48Snsolder no notable difference is found in the elemental distribution dueto the presence or absence of residual flux (XMA analysis).

The following considerations are made in light of the above.

i) At the time of soldering, the In and Sn atoms in the solder reactwith the carboxyl groups (--COOH) in the rosin (flux) to producecarboxylic acid salts (rosin--COOIn, rosin--COOSn) which are insolublein IPA. At such time the amounts of In and Sn on the surface of thesolder are reduced.

ii) If the heating tim (soldering) time is extended, the production ofcarboxylic acid salts also increases causing them to be leftover aspost-cleaning residue.

iii) Since the cause of the production of the residue are the In and Sncarboxylic acid salts, it is thought to be effective to add an "acid" toone of the components of a new defluxing agent. The "acid" dissociatesthe In ions and Sn ions from the In carboxylic acid salt and Sncarboxylic acid salt during cleaning, to return the metal carboxylicacid salt to carboxylic acid and thus facilitate the cleaning(dissolution).

Example 1

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acetic acid/isopropanol/xylene =1/2/7 (weight ratio,same hereunder) at room temperature for 10 minutes. When observation wasthen made for the presence of flux residue production, no residue wasfound.

Comparison 1

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of isopropanol/xylene =3/7 at room temperature for 30minutes. When observation was then made for the presence of flux residueproduction, a large amount of flux residue was found.

Comparison 2

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 3 minutes. After the heating, it was immersed in acommercially available TH-20A defluxing agent (Tokuyama Soda) at roomtemperature for 10 minutes. When observation was then made for thepresence of flux residue production, no flux residue was found. Then,when the heating time of the solder was lengthened to 30 minutes and anevaluation made in the same manner, a large amount of flux residue wasfound. FIG. 2A shows the appearance of this substrate after the cleaningwith TH-20A.

Example 2

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acrylic acid/isopropanol/xylene =1/2/7 at roomtemperature for 10 minutes. When observation was then made for thepresence of flux residue production, no residue was found.

Example 3

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of propionic acid/isopropanol/xylene =1/2/7 at roomtemperature for 10 minutes. When observation was then made for thepresence of flux residue production, no residue was found.

Example 4

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of benzoic acid/isopropanol =2/8 at 60° C. for 10minutes. When observation was then made for the presence of flux residueproduction, no residue was found.

Example 5

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acetic acid/isopropanol/benzyl acetate =1/2/7 at roomtemperature for 10 minutes. When observation was then made for thepresence of flux residue production, no residue was found.

Example 6

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acetic acid/isopropanol/methyl α-hydroxyisobutyrate=3/2/5 at room temperature for 10 minutes. When observation was thenmade for the presence of flux residue production, no residue was found.

Example 7

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acetic acid/isopropanol/xylene ⁼ 1/2/7 to which 1%sodium phosphate had been added, at room temperature for 10 minutes.When observation was then made for the presence of flux residueproduction, no residue was found. Also, no damage to the soldered metalwas found upon this immersion in the defluxing agent.

Example 8

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (Tamurathereon, as applied thereon, after which it was heated inperfluorocarbon vapor at 215° C. for 30 minutes.

After the heating, it was immersed in a defluxing agent of xylene/methylβ-methoxyisobutyrate/cyclohexanone/acetic acid=70/15/15/2.5 at roomtemperature for 10 minutes.

When observation was then made for the presence of flux residueproduction, no residue was found. FIG. 2B shows the appearance of thissubstrate after the cleaning with the above defluxing agent.

Comparison 3

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of cyclohexanone at room temperature for 30 minutes.When observation was then made for the presence of flux residueproduction, a large amount of residue was found.

Example 9

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of xylene/methyl β-methoxyisobutyrate/cyclohexanone=70/15/15 at room temperature for 30 minutes. When observation was thenmade for the presence of flux residue production, no residue was found.This combination of xylene/methyl β-methoxyisobutyrate/cyclohexanone=70/15/15 exhibited a highest cleaning power among solvents withoutaddition of an acid thereto.

Example 10

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of isoamyl acetate/butyl acetate/methylβ-methoxyisobutyrate/acetic acid =70/20/10/3 at 60° C. for 15 minutes.When observation was then made for the presence of flux residueproduction, no residue was found.

Example 11

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of dibenzyl ether/tetralin/methylβ-methoxyisobutyrate/acetic acid =65/30/5/1 (inflammation point approx.72° C.) at room temperature for 15 minutes. When observation was thenmade for the presence of flux residue production, no residue was found.

Example 12

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of benzyl acetate/tetralin/methylβ-methoxyisobutyrate/acetic acid =65/30/5/1 at room temperature for 15minutes. When observation was then made for the presence of flux residueproduction, no residue was found.

Example 13

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of dodecylbenzene/benzyl acetate/tetralin/methylβ-methoxyisobutyrate/acetic acid =35/20/40/5/2 at room temperature for15 minutes. When observation was then made for the presence of fluxresidue production, no residue was found.

Example 14

Examples 14 and 15 are defluxing agents containing no acid.

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 20 minutes.

After the heating, it was immersed in a defluxing agent of isoamylacetate/butyl acetate/methyl β-methoxyisobutyrate =70/20/10 at 60° C.for 30 minutes.

When observation was then made for the presence of flux residueproduction, no residue was found.

Example 15

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes.

After the heating, it was immersed for 10 minutes at room temperature ina defluxing agent of xylene/cyclohexanone/methylβ-methoxyisobutyrate/acetic acid =70/15/15/2.5 to which molecular sieve(4A) had been added to 10 wt %.

When observation was then made for the presence of flux residueproduction, no residue was found. Also, no damage to the soldered metalwas found upon this immersion in the defluxing agent.

Example 16

Examples 16-21 are instances of rinsing after cleaning.

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes.

This was immersed in a defluxing agent of aceticacid/isopropanol/xylene=1/20/79 at room temperature for 20 minutes,after which it was immersed in xylene for 5 minutes, further immersed infresh xylene for 5 minutes, and finally exposed to isopropyl alcoholvapor for 5 minutes.

When observation was then made for the presence of flux residueproduction, no residue was found. Also, this sample was placed in ahigh-temperature/high humidity layer for one week, but no corrosion ofthe metal parts was found.

Comparison 4

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. This was immersed in a defluxing agentof acetic acid/isopropanol/xylene=1/20/79 at room temperature for 20minutes, after which it was placed in a high-temperature/high-humiditylayer for one week. This resulted in observable corrosion of the metalparts.

Example 17

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. This was immersed in a defluxing agentof propionic acid/isopropanol/xylene=1/20/79 at room temperature for 20minutes, after which it was immersed in xylene for 10 minutes. Whenobservation was then made for the presence of flux residue production,no residue was found. Also, this sample was placed in ahigh-temperature/high-humidity layer for one week, but no corrosion ofthe metal parts was found.

Example 18

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. This was immersed in a defluxing agentof acetic acid/cyclohexanone/xylene=1/20/79 at room temperature for 20minutes, after which it was immersed in isopropyl alcohol for 10minutes. When observation was then made for the presence of flux residueproduction, no residue was found. Also, this sample was placed in ahigh-temperature/high-humidity layer for one week, but no corrosion ofthe metal parts was found.

Example 19

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. This was immersed in a defluxing agentof acetic acid/methyl β-methoxyisobutyrate/xylene =1/20/79 at roomtemperature for 20 minutes, after which it was immersed in butyl acetatefor 10 minutes. When observation was then made for the presence of fluxresidue production, no residue was found. Also, this sample was placedin a high-temperature /high-humidity layer for one week, but nocorrosion of the metal parts was found.

Example 20

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. After the heating, it was immersed in adefluxing agent of acetic acid/methyl α-hydroxyisobutyrate/anisole=3/20/80 at room temperature for 10 minutes, after which it was immersedin xylene for 10 minutes and further immersed in isopropyl alcohol for 5minutes.

When observation was then made for the presence of flux residueproduction, no residue was found. Also, this sample was placed in ahigh-temperature/high-humidity layer for one week, but no corrosion ofthe metal parts was found.

Comparison 5

Sn-37Pb, In-48Sn and In-40Pb solder was placed on a Au metallizedportion on a substrate and commercially available Flux MH-820V (TamuraKaken) was applied thereon, after which it was heated in perfluorocarbonvapor at 215° C. for 30 minutes. This was immersed in a defluxing agentof acetic acid/isopropanol/xylene=1/20/79 at room temperature for 20minutes, after which it was immersed in perfluorocarbon (boiling pointapprox. 100° C.) for 5 minutes. It was then placed in a high-temperature/high-humidity layer for one week. This resulted in observable corrosionof the metal parts.

Example 21

In FIG. 3A shows an overall view of a cleaning apparatus which comprisesa wash basin 7 filled with cleaning liquid, a rinse basin 8 for rinsingthe defluxing agent, and a conveying mechanism 6 for conveying theobject to be washed (a circuit wiring board 4), and the inside of theapparatus is an N₂ atmosphere. FIG. 3B is a drawing which shows theinside of the cleaning basin 7, where 1 is the defluxing agent, 2 is N₂gas, 3 is a layer of floating bodies, 4 is a soldered circuit wiringboard (object to be cleaned) and 5 is a conveying jig.

A mixed solution of xylene 70/cyclohexanone 15/methylβ-methoxyisobutyrate/acetic acid 2.5 was placed in the cleaning basin ofthe defluxing apparatus, two types of polyethylene hollow balls (φ20 mm,φ50 mm) were floated on the surface of the solution, and the circuitboard was cleaned. When the circuit board was pulled out of the cleaningbasin, the hollow balls were not taken out with it.

After 10 hours, a different circuit board was cleaned, and when bothboards were compared, no differences were found in them.

In this Example, it is thought that the substance with no affinity forthe defluxing agent, which was floated on the surface of the defluxingagent 1 to isolate it from the outside air, minimized evaporation(vaporization) of the defluxing agent and thus prevented changes in thecomposition of the agent.

In order to confirm this, a solution of xylene 70/cyclohexanone15/methyl β-methoxyisobutyrate/acetic acid 2.5 was placed in a containerwith a round bottom of diameter 78 mmφ and a height of 86 mm, and acomparison was made between the weight reduction by evaporation of asolution on whose surface 10 mmφ spheres were floated and one on whosesurface they were not. The results are shown in FIG. 4.

Example 22

FIGS. 5-8 show cleaning apparatuses. FIG. 5 is a front cross-sectionalview and FIG. 6 is a side cross-sectional view.

A cleaning basin 11 containing a defluxing agent and a first rinse basin12 and a second rinse basin 13 containing rinsing solutions are arrangedinside an airtight surrounding case 14. For introduction of nitrogen gasinto the surrounding case 14, N₂ gas is introduced from an N₂ source viaa pressure-regulating valve 16 and an electromagnetic valve 17, into thecompartment through the holes of a perforated copper pipe situated nearthe inner roof of the surrounding case 14, while it is discharged via adischarge valve 19 out through a discharge duct 20. Since the organicsolvent of the defluxing agent and rinsing solution is generally heavierthan air, if the opening for discharge of the N₂ gas is provided at aposition lower than that of the opening for introduction of the N₂ gas,then efficient discharge of the solvent vapor will become possible, andthus the amount of the N₂ gas introduced may be reduced.

In this surrounding case, an object to be cleaned work 21 is placed in awash basket which is hung by a chain 23 and can be moved forward andbackward, and up and down, by a motor 25 along a beam 24 equipped with aguide which runs across it near the roof. The beam 24 is supported by asupporting leg frame 26.

The apertures for the supply and removal of the objects to be cleanedinto and out of the surrounding case 14 must be opened and closed, andthis causes a problem since air entering the surrounding case 14 raisesthe oxygen concentration, and running costs are thus increased as extratime and N₂ gas must be consumed to lower the oxygen concentration.Here, a front compartment 29 and a rear compartment 30 are providedoutside of the loading aperture 27 and unloading aperture 28, and N₂ isalso flow into these front and rear compartments 29, 30 which are usedas buffers to overcome the above mentioned problem.

Furthermore, in order to quickly lower the oxygen concentration toshorten the operating time of the cleaning apparatus it is necessary tosupply/discharge nitrogen gas at a large volume, but the constant flowof a large volume of nitrogen is not economically feasible. Here, bydetecting the oxygen concentration in the enclosed space and reducing(or stopping) the nitrogen flow when the oxygen concentration becomessufficiently low, and then increasing the flow when the oxygenconcentration increases once again, either the intake or outlet, orboth, of the nitrogen gas may be controlled to reduce the amount ofnitrogen gas used. The same control may be effected by detecting theconcentration of the organic solvent gas instead of the oxygenconcentration.

FIG. 7 shows a situation in which an oxygen concentration sensor 35 isused to detect the oxygen concentration in the surrounding case 14, andelectromagnetic valves 37, 38 are operated by a microcontroller 36 inresponse to the detected oxygen concentration to adjust the flow of theN₂ gas introduced into the surrounding case 14. For example, when theflow of N₂ gas of 2 kgf/cm² passing through electromagnetic valve 37 isadjusted to 10 l/min and that passing through electromagnetic valve 38to 100 1/min, the O₂ concentration inside the surrounding case 14 willbe 3 vol % and 5 vol %, respectively.

FIG. 8 shows the inside of the pressure-regulating valve at thedischarge end. The gas inside the surrounding case 14 passes through adischarge duct 41 to reach a discharge valve 42, and finally to reach adischarge disposal section 43. The discharge valve 42 is rotatablearound a pivot 44, and the valve opens and closes by the balance betweenits own weight and the force of the gas pushing on the valve from out ofthe surrounding case 14. When such a discharge valve was designed andoperated so that the pressure inside the surrounding case 14 was 5 hPagreater than the outside air, the pressure differential between theinside and outside of the surrounding case actually became 2-3 hPa as aresult of back pressure of the discharge and leaks in the compartment asa whole. In FIG. 7, F₁ indicates the closing force due to the weight ofthe discharge valve, and F₂ indicates the raising force of the gaspushing on the valve.

As explained above, with a defluxing agent according to the presentinvention, satisfactory defluxing is possible without leaving corrosivesubstances in the soldered portion, thus improving the reliability ofelectronic circuit components. Also, there is none of the globalenvironmental destruction which occurs with the conventionalchlorine-type defluxing agents.

Furthermore, since the placing of floating bodies on the cleaning bathmakes it possible to prevent changes in the composition of the defluxingagent which is a mixture of a plurality of organic solvents, stabledefluxing of a circuit wiring board may be accomplished. It is thuspossible to ensure the long-term reliability of electronic devices.

In addition, simplification of the apparatus is made possible bycontrolling the pressure differential between the inside and outside ofthe cleaning apparatus into which N₂ gas has been introduced to lowerthe concentration of the inflammable substances.

We claim:
 1. A method comprising the steps of:(a) soldering a member toa substrate using flux, (b) cleaning the flux remaining on the substratewith a first liquid, the first liquid being a defluxing agent containingan acid and an organic solvent and containing substantially no water,theorganic solvent being a mixture containing 50-95 parts by weight of anaromatic solvent having 6-20 carbon atoms, and 5-50 parts by weight ofat least one solvent selected from the group consisting of estersolvents having 3-15 carbon atoms, ketone solvents having 2-15 carbonatoms and alcohol solvents having 1-15 carbon atom, and the organicsolvent contains substantially no chlorine or fluorine, and (c) cleaningthe substrate with a second liquid which contains no acid and readilymixes with said acid-containing defluxing agent, to remove the acid. 2.A method according to claim 1 which uses, as said second liquid, atleast one compound selected from the group consisting of aromaticcompounds, hydroxy compounds, ketones, ethers and esters whose boilingpoint at one atmospheric pressure is between 50° C. and 250° C. andwhich contain no fluorine or chlorine.
 3. A method according to claim 1which uses water as said second liquid.
 4. A method according to claim 1which further contains the step of, after the cleaning with said secondliquid, further cleaning with a third liquid which readily mixes withsaid second liquid, to remove the acid.
 5. A method according to claim 1which further contains the step of, after the cleaning with said secondliquid, further cleaning with vapor of the second liquid or a thirdliquid which readily mixes with the second liquid, to remove the acid.6. A method according to claim 1, wherein the second liquid and theorganic solvent are the same solvent.
 7. A method according to claim 1,wherein the acid in the defluxing agent is an organic acid selected fromthe group consisting of acrylic acid, acetic acid, propionic acid andbenzoic acid.
 8. A method according to claim 1, wherein the organicsolvent has a boiling point of 50°-250° C. at one atmosphere ofpressure.
 9. A method comprising the steps of:(a) soldering a member toa substrate using flux, (b) cleaning the flux remaining on the substratewith a first liquid, the first liquid being a defluxing agent containingan acid and an organic solvent and containing substantially no water,the organic solvent being a mixture containing 50-95 parts by weight ofa substance consisting of at least one of butyl acetate and amylacetate, and 5-50 parts by weight of at least one solvent selected fromthe group consisting of ester solvents having 3-15 carbon atoms, ketonesolvents having 2-15 carbon atoms and alcohol solvents having 1-15carbon atoms, andthe organic solvent contains substantially no chlorineor fluorine, and (c) cleaning the substrate with a second liquid whichcontains no acid and readily mixes with said acid-containing defluxingagent, to remove the acid.
 10. A method according to claim 9 which uses,as said second liquid, at least one compound selected from the groupconsisting of aromatic compounds, hydroxy compounds, ketones, ethers andesters whose boiling point at one atmospheric pressure is between 50° C.and 250° C. and which contain no fluorine or chlorine.
 11. A methodaccording to claim 9, which uses water as said second liquid.
 12. Amethod according to claim 9 which further contains the step of, afterthe cleaning with said second liquid, further cleaning with a thirdliquid which readily mixes with said second liquid, to remove the acid.13. A method according to claim 9 which further contains the step of,after the cleaning with said second liquid, further cleaning with vaporof the second liquid or a third liquid which readily mixes with thesecond liquid, to remove the acid.
 14. A method according to claim 9,wherein the second liquid and the organic solvent are the same solvent.15. A method according to claim 9, wherein the acid in the defluxingagent is an organic acid selected from the group consisting of acrylicacid, acetic acid, propionic acid and benzoic acid.
 16. A methodaccording to claim 9, wherein the organic solvent has a boiling point of50°-250° C. at one atmosphere of pressure.
 17. A method comprising thesteps of:(a) soldering a member to a substrate using flux, (b) cleaningthe flux remaining on the substrate with a first liquid, the firstliquid being a defluxing agent containing an acid and an organic solventand containing substantially no water, the organic solvent being amixture containing 50-90 parts by weight of xylene, 0.5-10 parts byweight of acetic acid and 10-50 parts by weight of a substanceconsisting of at least one of cyclohexanone and methylβ-methoxyisobutyrate, and (c) cleaning the substrate with a secondliquid which contains no acid and readily mixes with saidacid-containing defluxing agent, to remove the acid.
 18. A methodaccording to claim 17 which uses, as said second liquid, at least onecompound selected from the group consisting of aromatic compounds,hydroxy compounds, ketones, ethers and esters whose boiling point at onatmospheric pressure is between 50° C. and 250° C. and which contain nofluorine or chlorine.
 19. A method according to claim 17, which useswater as said second liquid.
 20. A method according to claim 17 whichfurther contains the step of, after the cleaning with said secondliquid, further cleaning with a third liquid which readily mixes withsaid second liquid, to remove the acid.
 21. A method according to claim17 which further contains the step of, after the cleaning with saidsecond liquid, further cleaning with vapor of the second liquid or athird liquid which readily mixes with the second liquid, to remove theacid.
 22. A method according to claim 17, wherein the second liquid andthe organic solvent are the same solvent.
 23. A method according toclaim 17, wherein the acid in the defluxing agent is an organic acidselected from the group consisting of acrylic acid, acetic acid,propionic acid and benzoic acid.
 24. A method according to claim 17,wherein the organic solvent has a boiling point of 50°-250° C. at oneatmosphere of pressure.